Control circuit for an electromagnetic drive

ABSTRACT

A control circuit for an electromagnetic drive includes first and second electronic switching elements, which in conjunction with a timing element subject the drive coil to a corresponding direct current in the starting phase or in the maintenance phase. A starting current and a maintenance current are provided by means of a current source that is controlled by the timing element and a direct current converter with downward control.

The present invention relates to a control circuit for anelectromagnetic operating mechanism, in particular, the operatingmechanism of an electromagnetic switching device. The electromagneticoperating mechanism generally includes an operating coil, a magneticcore, and an armature.

BACKGROUND

An electronic drive control for a contactor operating mechanism isdescribed in German Publication DE 299 09 901 U1. The drive controlessentially includes a rectifier circuit supplied via control inputs, aseries circuit which is composed of the operating coil and a pulse-widthcontrolled transistor switch and is supplied by the rectifier circuit,two voltage divider circuits which scan the output of the rectifiercircuit and are isolated on the input side by an isolation diode, aswell as an electronic circuit including a microprocessor and twomemories. Control signals for the pickup and holding modes of theoperating coil are supplied to the transistor by the electronic circuit;the corresponding pulse widths in the pickup and holding modes beingdetermined via the associated memory in accordance with the outputsignal of the associated voltage divider. Moreover, it is known fromGerman Publication DE 299 09 904 U1 to provide such electronic drivecontrols with a first transistor switch for controlling the pickupcurrent and a second transistor switch for controlling the holdingcurrent. Such electronic drive controls have the disadvantage of havinga high degree of complexity, which is due to the electronic circuit andis of particular consequence for operating mechanisms of lower-ratedelectromagnetic switching devices.

German Publication DE 92 16 041 U1 describes a circuit arrangement forcontrolling a relay. The series circuit of the operating coil and thefirst transistor switch is connected to a DC operating voltage, and theseries circuit of a holding resistor and a second transistor switch isplaced in parallel with the switching path of the first transistorswitch. A d.c. control input is connected, via a differentiating timerincluding a capacitor and a discharge resistor, to the control electrodeof the first transistor switch and, via a series resistor, to thecontrol electrode of the second transistor switch. After a controlvoltage has been applied, both the first and second transistor switchesare turned on, as a result of which a pickup voltage is applied acrossthe operating coil; the pickup voltage obtained being the DC operatingvoltage minus the saturation voltage of the first transistor switch.When the capacitor voltage of the differentiating element has dropped,the first transistor goes to the OFF state. Consequently, the operatingcoil is then only supplied with a holding current, which is essentiallyobtained from the ratio of the DC operating voltage to the sum of theholding resistance and the ohmic resistance of the operating coil. Afterthe control voltage has been removed, the second transistor switch isalso turned off, thereby switching off the relay. In the case of thiscontrol circuit, both the pickup response and the reliability and heatlosses in the holding mode are highly dependent on changes andfluctuations in the DC operating voltage. The drive control, which isonly suitable for DC voltage operation, uses a control voltage inaddition to the operating voltage; the control voltage being independentof the operating voltage. An additional significant amount of power islost through the holding resistor.

German Patent DE 44 10 819 C2 discloses a circuit arrangement which isintended to operate a relay and which, in turn, has a first transistorswitch, which is turned on during the pickup phase, and a secondtransistor switch, which is placed in series with the operating coil anda holding resistor and connected to an operation voltage and which isturned on when the relay is in the ON state. The switching path of thefirst transistor switch is placed in parallel with the holding resistor.A d.c. control input is connected via a voltage divider to the controlelectrode of the second transistor switch. The control electrode of thefirst transistor switch is connected to the junction point of the firsttransistor switch, the second transistor switch and the holding resistorvia an integrating timer including a charging resistor and a capacitor.When the relay is in the OFF state, the capacitor is charged via theoperating coil, the holding resistor and the charging resistor so thatboth transistor switches are turned on when a control voltage isapplied. In this connection, the pickup voltage obtained for theoperating coil equals the operating voltage minus the sum of thesaturation voltages of the two transistor switches. At the same time,the capacitor begins to discharge through the series resistor and theswitching path of the second transistor switch. After the capacitorvoltage has fallen below a threshold value, the first transistor isturned off. Consequently, the operating coil is then only supplied witha holding current, which is essentially obtained from the ratio of theDC operating voltage to the sum of the holding resistance and the ohmicresistance of the operating coil. After the control voltage has beenremoved, the second transistor switch is also turned off, therebyswitching off the relay. This drive control presents the above-describeddisadvantages of the approach of German Publication DE 92 16 041 U1 andrequires an operating voltage to be provided continuously or at leastwith sufficient time before the relay is switched on.

German Patent 196 38 260 C2 discloses a circuit arrangement forcontrolling small solenoid coils, including a transistor switchconnected in series with the solenoid coil. Upon application of acontrol voltage, the turned-on transistor switch applies a high pickupcurrent to the solenoid coil during a time period set by adifferentiating timer. After that, the holding current is determined bya series circuit which is composed of a holding resistor and alight-emitting diode and is placed in parallel with the switching pathof the transistor switch. Here too, the pickup and holding currents arehighly dependent on the magnitude of the control voltage, and asignificant amount of power is lost through the holding resistor.

SUMMARY OF THE INVENTION

It is therefore the an object of the present invention to provide alow-power control circuit that has a low degree of complexity and islargely independent of voltage.

The present invention provides a control circuit for an electromagneticoperating mechanism, wherein a pickup voltage and a holding voltage,which is significantly lower than the pickup voltage, are provided byrelatively simple means in the form of a timer-controlled voltage sourceand a step-down d.c. voltage converter. The magnitude of the pickupvoltage is below the permissible operating voltage range and is largelyindependent of the magnitude of the control voltage. The holding voltageis controlled to a level which, in terms of absolute value, is far belowthe pickup voltage. The voltage applied to the control input, which canbe selected to be a DC voltage or an AC voltage, at the same time powersthe control circuit. After the control voltage has been applied, theoperating voltage is built up immediately via the rectifier circuit. Thedeveloping operating voltage, first of all, activates a timer and buildsup the holding voltage via the d.c. voltage converter. The operatingcoil is energized by the activated voltage source via the firstswitching means, while the switching path of the second switching means,which is placed in series with the operating coil, is enabledconcurrently. An isolation diode prevents the pickup voltage fromreaching the output of the d.c. voltage converter. After a certain timehas elapsed, that is, after the pickup time has elapsed, the timerdeactivates the voltage source and thereby also the first switchingmeans. Power supply to the operating coil as well as the maintained ONstate of the second switching means are then provided by the d.c.voltage converter with the holding voltage supplied via the isolationdiode. After the control voltage has been removed, the operating voltageand the holding voltage break down, whereupon the second switching meansare turned off, as a result of which the operating coil is de-energized.The time behavior of the timer and the pickup voltage must be selectedsuch that the armature activated by the operating coil is reliablyattracted by the magnetic core. During the holding phase, the voltageacross the operating coil is significantly lower than during the pickupphase. The holding voltage must be selected, by adjusting the d.c.voltage converter, to a level just sufficient to reliably hold thearmature in its attracted position.

The proposed control circuit does not need any complex digital means,especially no microcontroller, and is suitable for both DC and ACoperating mechanisms, and especially for lower-power electromagneticoperating mechanisms. Since the pickup time and the holding current canassume low values, the control circuit of the present invention alsoallows the use of AC electromagnetic operating mechanisms that havelow-resistance operating coils and which, without using the proposedcontrol circuit, would otherwise only be suitable for AC operation. Thisallows the manufacture of electromagnetic switching devices to belimited to only AC operating mechanisms, thereby making it possible toreduce the necessary operating coil variants, and thus to markedlyreduce costs.

The timer can advantageously be implemented as a simple, integrating ordifferentiating RC element (also referred to as a “low-pass filter” or“high-pass filter”). The combination with a voltage-limiting device, forexample, a Zener diode, results in a limitation of the charging endvoltage, thereby considerably reducing the dependence of the chargingand discharging processes on the magnitude of the operating voltage.

The controllable voltage source includes a voltage-limiting circuitcombined with a threshold circuit and is therefore inexpensive. Whenusing an integrating timer, usually, the charge voltage increasing atthe charging capacitor of the RC element is evaluated by the thresholdswitch as the controlling value for the termination of the pickup phase.When using a differentiating timer, the threshold switch usuallyevaluates the voltage decreasing at the discharge resistor as a resultof the discharging current.

Free-wheeling means, such as a Zener diode, which are placed in parallelwith the switching path of the second switching means, provide a fastdemagnetization of the operating coil during de-energization, possiblyin cooperation with other free-wheeling means.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details and advantages of the present invention will becomeapparent from the exemplary embodiment described below with reference tothe Figures, in which:

FIG. 1 is a schematic representation of the control circuit according tothe present invention;

FIG. 2 is a detailed view of an advantageous embodiment of the controlcircuit of the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a control circuit 2 for an operating coil 4 of anelectromagnetic operating mechanism (not specifically shown) of anelectromagnetic switching device; the control circuit being operated bya control voltage Ue via a control input 6. The control voltage Ueapplied can optionally be a DC voltage or an AC voltage. When controlvoltage Ue is applied, a smoothed operating voltage Ub is present at theoutput of a rectifier circuit 8; the smoothed operating voltage beingused, inter alia, for power supply to control circuit 2 and to operatingcoil 4. A d.c. voltage converter 10 downstream of rectifier circuit 8converts operating voltage Ub to a significantly lower smoothed holdingvoltage Uh. After control voltage Ue has been applied, the rapidlyincreasing operating voltage Ub triggers a timer 12, the time behaviorof which controls the duration of the pickup phase of control circuit 2.Triggered timer 12 activates a voltage source 14 which, when in theactivated state, provides at its output a pickup voltage Ua, which isderived from operating voltage Ub. The magnitude of pickup voltage Ua isbelow that of the minimum permissible operating voltage Ub, and islargely independent of operating voltage Ub within a wide range thereof.Pickup voltage Ua activates first electronic switching means 16 whichact as a voltage follower and whose output is connected to firstterminal 18 of operating coil 4. Thus, during the pickup phase, firstterminal 18 of operating coil 4 is at a potential which, due to acomponent-related saturation voltage of first switching means 16,differs only slightly from pickup voltage Ua. The output of firstswitching means 16 is further connected to the control input of secondelectronic switching means 22 whose switching path leads from secondterminal 20 of operating coil 4 to the reference potential of operatingvoltage Ub. Pickup voltage Ua causes the switching path of secondswitching means 22 to be enabled. Thus, during the pickup phase,operating coil 4 is supplied with a voltage whose magnitude is slightlyreduced by the saturation voltages of the two switching means 16 and 22as compared to pickup voltage Ua. The output of d.c. voltage converter10 is connected to the output of first switching means 16 via anisolation diode 24 in the forward direction. During the pickup phase,isolation diode 24 is blocked because the magnitude of pickup voltage Uais significantly higher than that of holding voltage Uh.

At the end of the pickup phase, the output signal of timer 12 haschanged to the point where pickup voltage Ua, which has been present atthe output of voltage source 14, is turned off. Because of this, thevoltage at the output of first switching means 16 decreases to such alevel that holding voltage Uh now reaches first terminal 18 of operatingcoil 4 and the control input of second switching means 22 via isolationdiode 24. Thus, the holding phase has begun. During the holding phase,operating coil 4 is supplied with a voltage whose magnitude is reducedonly by the saturation voltages of conducting isolation diode 24 and ofthe enabled switching path of second switching means 22 as compared toholding voltage Uh.

After control voltage Ue has been removed from input 6 of controlcircuit 2, operating voltage Ub and holding voltage Uh break downquickly. Thus, the two switching means 16, 22 assume the OFF state,whereupon operating coil 4 is de-energized.

FIG. 2 illustrates a detailed advantageous embodiment of above-describedcontrol circuit 2. The reference numerals used in FIG. 1 for thefunctional groups have been adopted here.

As is usual, rectifier circuit 8 includes a limiter device 28 on theinput side, a bridge rectifier 26, and a first smoothing capacitor 30.After control voltage Ue has been applied, operating voltage Ub hasramped up in a short period of time. When driving and operating thecontrol circuit with a control voltage Ue in the form of a DC voltage,bridge rectifier 26 serves as a reverse polarity protection.

Timer 12 is designed as an integrating RC element. Starting at a supplyline 32 carrying operating voltage Ub, a charging current flows throughthe series circuit of two charging resistors 34 and 36 to a chargingcapacitor 38 after operating voltage Ub has appeared. The voltage at afirst junction point 40 of the two charging resistors 34, 36 is limitedby a voltage-limiting device in the form of a Zener diode 42. Thus, thetime behavior of timer 12 is largely independent of the magnitude ofoperating voltage Ub. The time behavior is mainly determined by thedesign of the RC element formed by charging resistor 36 and chargingcapacitor 38. After control voltage Ue has been removed, chargingcapacitor 38 discharges through a discharge resistor 44 and a dischargediode 46 into the now de-energized supply line 32. Thus, timer 12 isready to be turned on again.

Controllable voltage source 14 is includes a threshold circuitevaluating the charge voltage of charging capacitor 38 and avoltage-limiting circuit coupled to the output of the threshold circuit.The voltage-limiting circuit is formed by a series circuit of a firstseries resistor 48 and a series of Zener diodes 50, and is placedbetween supply line 32 and the reference potential. The thresholdcircuit features a third transistor 52 in common source configuration.Charging capacitor 38 is connected via a second Zener diode 54 to thegate terminal of third transistor 52. A bleed resistor 56 placed betweenthe gate terminal of third transistor 52 and the reference potential isused to protect the gate electrode. The drain terminal of thirdtransistor 52 is connected via a load resistor 58 to a second junctionpoint 60, which is common to first series resistor 48 and the series ofZener diodes 50. As long as the voltage across charging capacitor 38 hasnot yet exceeded the sum of the Zener voltage of second Zener diode 54and the switching threshold of the gate voltage of third transistor 52,third transistor 52 is in the OFF or non-conducting state. In this case,pickup voltage Ua is present at second junction point 60; the pickupvoltage being derived from the sum of the Zener voltages of the seriesof Zener diodes 50. When, toward the end of the pickup phase, thevoltage at charging capacitor 38 exceeds the sum of the Zener voltage ofsecond Zener diode 54 and the switching threshold of the gate voltage ofthird transistor 52, the third transistor goes to the ON or conductingstate. In this case, the voltage at second junction point 60 falls farbelow pickup voltage Ua. The resistance value of series resistor 48 isselected to be high compared to that of load resistor 58.

First switching means 16 are formed by a first transistor 62 in sourcefollower configuration with a first protective diode 64 to protect firsttransistor 62 from negative voltage spikes between the gate and sourceterminals thereof. The output of first switching means 16, which isconnected to first terminal 18 of operating coil 4, is identical to thesource terminal of first transistor 62 and, during the pickup phase,supplies pickup voltage Ua, which is reduced by the gate-source voltageof first transistor 62. Due to the potential drop at second junctionpoint 60 toward the end of the pickup phase, first transistor 62 isturned off.

D.c. voltage converter 10 is formed by a converter circuit 66 connectedat the input to supply line 32, by smoothing means on the output side,as well as detecting means for measuring and controlling the outputholding voltage Uh. As is usual, the smoothing means are formed by asmoothing choke 68 and a feedback diode 70 at the output of convertercircuit 66 as well as a second smoothing capacitor 72 connecteddownstream of smoothing choke 68. When control voltage Ue is applied,holding voltage Uh is present across second smoothing capacitor 72. Thedetecting means are formed by a series circuit which is composed of athird Zener diode 74 and a photodiode 76 and is placed in parallel withsecond smoothing capacitor 72, and by a phototransistor 78 opticallycoupled to photodiode 76. Phototransistor 78 is connected at its emitterterminal to the output of converter circuit 66 and at its collectorterminal to a control input of the converter circuit. Thus, holdingvoltage Uh is determined by the sum of the Zener voltage of third Zenerdiode 74 and the conducting-state voltage of photodiode 76. Aftercontrol voltage Ue has been applied, holding voltage Uh has ramped up inabout 30 ms. After control voltage Ue has been removed, second smoothingcapacitor 72 discharges in a short period of time through the currentpath formed by isolation diode 24, operating coil 4, and the switchingpath of second switching means 22.

Second switching means 22 include a second transistor 80 in commonsource configuration. This second transistor is connected to firstterminal 18 of operating coil 4 through a second series resistor 82, andto a second protective diode 84. Second protective diode 84 is designedas a Zener diode and protects the gate terminal of second transistor 80from excessive voltages, especially during the pickup phase. The drainterminal of second transistor 80 is connected to second terminal 20 ofoperating coil 4. During the pickup phase, second transistor 80 isswitched to the ON or conducting state due to pickup voltage Ua from theoutput of first switching means 16, and during the holding phase due toholding current Uh via conducting isolation diode 24, so that operatingcoil 4 is continuously energized during both phases. When controlvoltage Ue is absent or removed, second transistor 80 is in the OFF ornon-conducting state, thus preventing operating coil 4 from beingcontinuously energized. A free-wheeling means 86, which in the exampleis a Zener diode, is placed in parallel with the switching path ofsecond transistor 80. During both the pickup phase and the holdingphase, free-wheeling means 86 is short-circuited by the enabledswitching path of second transistor 80 and, therefore, has no effect.However, when second transistor 80 is turned off, operating coil 4discharges in a short period of time through the current path formed byfree-wheeling means 86, feedback diode 70, smoothing choke 68, andisolation diode 24. The relatively high free-wheeling voltage mainlycaused by the Zener voltage of free-wheeling means 86 causes themagnetic energy stored in operating coil 4 to be quickly removed,thereby causing the electromagnetic operating mechanism to be quicklyturned off.

The present invention is not limited to the embodiment described above.For example, the present invention can also be implemented using adifferentiating timer, such as is described, for example, in GermanPublication DE 92 16 041 U1 mentioned at the outset.

1. A control circuit for an electromagnetic operating mechanism, thecontrol circuit comprising: a timer; a first electronic switching deviceincluding a voltage follower and including a first output connected inseries with an operating coil of the electromagnetic operatingmechanism, the first electronic switching device being configured toactivate for a duration of a pickup phase of the electromagneticoperating mechanism after a control voltage has been applied via thetimer; a second electronic switching device including a switching pathconnected in series with the operating coil, the second electronicswitching device being turned on while the control voltage is present; arectifier circuit connected to a control input, the rectifier circuitincluding a second output and being configured to supply a smoothedoperating voltage at the second output; a step-down DC voltage converterconnected downstream of the rectifier circuit, the step-down DC voltageconverter including a third output and being configured to supply asmoothed holding voltage at the third output; and a voltage sourcecontrollable by the timer and configured to activate the firstelectronic switching device by a pickup voltage; wherein: the timer isactivatable by a ramping up of the operating voltage; the operating coiland the switching path of the second electronic switching device form aseries circuit connected to the first output; the series circuit and thefirst electronic switching device are suppliable with the operatingvoltage; and the third output, the first output, and a control input ofthe second electronic switching device are interconnected, the thirdoutput being interconnected via a forward biased isolation diode.
 2. Thecontrol circuit as recited in claim 1 wherein the electromagneticswitching device includes an operating mechanism.
 3. The control circuitas recited in claim 1 wherein the timer includes an integrating RCelement.
 4. The control circuit as recited in claim 1 wherein the timerincludes a differentiating RC element.
 5. The control circuit as recitedin claim 3 wherein the RC element is combined with a voltage-limitingdevice.
 6. The control circuit as recited in claim 4 wherein the RCelement is combined with a voltage-limiting device.
 7. The controlcircuit as recited in claim 1 wherein the voltage source includes avoltage-limiting circuit and a threshold circuit having an input side,the voltage-limiting circuit being supplied with the operating voltageand having a fourth output operatively connected to a switching path ofa the threshold circuit, the input side of the threshold circuit beingconnected to the timer.
 8. The control circuit as recited in claim 1further comprising a free-wheeling device connected in parallel with theswitching path of the second electronic switching device.
 9. The controlcircuit as recited in claim 3 wherein the voltage source includes avoltage-limiting circuit and a threshold circuit having an input side,the voltage-limiting circuit being supplied with the operating voltageand having a fourth output operatively connected to a switching path ofa the threshold circuit, the input side of the threshold circuit beingconnected to the timer.
 10. The control circuit as recited in claim 4wherein the voltage source includes a voltage-limiting circuit and athreshold circuit having an input side, the voltage-limiting circuitbeing supplied with the operating voltage and having a fourth outputoperatively connected to a switching path of the threshold circuit, theinput side of the threshold circuit being connected to the timer. 11.The control circuit as recited in claim 5 wherein the voltage sourceincludes a voltage-limiting circuit and a threshold circuit having aninput side, the voltage-limiting circuit being supplied with theoperating voltage and having a fourth output operatively connected to aswitching path of the threshold circuit, the input side of the thresholdcircuit being connected to the timer.
 12. The control circuit as recitedin claim 3 further comprising a free-wheeling device connected inparallel with the switching path of the second electronic switchingdevice.
 13. The control circuit as recited in claim 4 further comprisinga free-wheeling device connected in parallel with the switching path ofthe second electronic switching device.
 14. The control circuit asrecited in claim 5 further comprising a free-wheeling device connectedin parallel with the switching path of the second electronic switchingdevice.